Johnny is suffering from brain damage

As we all know, Johnny is a bit off. He has been since around his first appearance way back in chapter 1, where Max lands on Johnny while going to school.

When Max hit Johnny in the head with his scooter, he's noticeably injured. Stephen even comments on it on the next page, where Johnny seems perfectly fine. This is likely because of the spirit that has attached itself to his head, who we later learn is a Doctopus, which can relieve pain.

You can never really know with comic mechanics... If someone landed on your face with their full weight like that, you'd probably be dead. Same for this right here. There's not even a doctopus there and he wasn't even unconscious.

Also, if his behaviour was off, his friends would've probably said something. It's really likely he was just always like that. Like, this example you gave, Ollie isn't even worried about how Johnny's acting, that's just how he always is... We've also seen his friends (well Ollie at least) aren't oblivious enough to not notice a change in his behaviour.

I think if Johnny was so brain dead to the point of not being affected by his brain being further damaged, then he would probably be a vegetable, and therefore would not be able to go to school and/or interact with other people the way he does.